Suppression of dark current in a photosensor for imaging

ABSTRACT

A pixel cell having a halogen-rich region localized between an oxide isolation region and a photosensor. The halogen-rich region prevents leakage from the isolation region into the photosensor, thereby suppressing dark current in imagers.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 10/653,152,filed on Sep. 3, 2003, now U.S. Pat. No. 7,064,406.

FIELD OF THE INVENTION

The present invention relates generally to an imaging device and morespecifically to an imaging device pixel cell having a halogen-richregion formed therein, suppressing dark current in a photosensor.

BACKGROUND OF THE INVENTION

Imaging devices, including charge-coupled-devices (CCD) andcomplementary metal oxide semiconductor (CMOS) sensors have commonlybeen used in photo-imaging applications.

Exemplary CMOS imaging circuits, processing steps thereof, and detaileddescriptions of the functions of various CMOS elements of an imagingcircuit are described, for example, in U.S. Pat. No. 6,140,630 toRhodes, U.S. Pat. No. 6,376,868 to Rhodes, U.S. Pat. No. 6,310,366 toRhodes et al., U.S. Pat. No. 6,326,652 to Rhodes, U.S. Pat. No.6,204,524 to Rhodes, and U.S. Pat. No. 6,333,205 to Rhodes. Thedisclosures of each of the forgoing patents are hereby incorporated byreference in their entirety.

An imager, for example, a CMOS imager includes a focal plane array ofpixel cells; each cell includes a photosensor, for example, a photogate,a photoconductor, or a photodiode overlying a substrate for producing aphoto-generated charge in a doped region of the substrate. A readoutcircuit is provided for each pixel cell and includes at least a sourcefollower transistor and a row select transistor for coupling the sourcefollower transistor to a column output line. The pixel cell alsotypically has a floating diffusion node, connected to the gate of thesource follower transistor. Charge generated by the photosensor is sentto the floating diffusion node. The imager may also include a transfertransistor for transferring charge from the photosensor to the floatingdiffusion node and a reset transistor for resetting the floatingdiffusion node to a predetermined charge level prior to chargetransference.

A conventional pixel cell 10 of an image sensor, such as a CMOS imager,is illustrated in FIG. 1. Pixel cell 10 typically includes a photosensor12 having a p-region 12 a and n-region 12 b in a p-substrate 14. Thep-region 12 a of the photosensor 12 is typically coupled to thepotential of the p-substrate 14 for efficient operation of thephotosensor 12. The pixel cell 10 also includes a transfer transistorwith associated gate 16, a floating diffusion region 18 formed in a moreheavily doped p-type well 20, and a reset transistor with associatedgate 22. The reset transistor 22 has an associated source/drain region30 that is used when resetting the floating diffusion region 18 to apredetermined charge level prior to charge transference.

Photons striking the surface of the p-region 12 a of the photosensor 12generate electrons that are collected in the n-region 12 b of thephotosensor 12. When the transfer gate 16 is on, the photon-generatedelectrons in the n-region 12 b are transferred to the floating diffusionregion 18 as a result of the potential difference existing between thephotosensor 12 and floating diffusion region 18. Floating diffusionregion 18 is coupled to the gates of a source follower transistor 24,which receives the charge temporarily stored by the floating diffusionregion 18 and transfers the charge to a first source/drain terminal of arow select transistor 26. When the row select signal RS goes high, thephoton-generated charge is transferred to the column line 28 where it isfurther processed by sample/hold and processing circuits (not shown).

Pixel cell 10 is typically formed between two isolation regions 32. Inthe illustrated pixel cell 10 the two isolation regions are shallowtrench isolation (STI) regions 32. The STI regions 32 prevent crosstalkbetween adjacent pixels, as pixel cell 10 is only one of hundreds orthousands of pixels in a pixel cell array. The pixel cell array istypically organized as rows and columns. Each row and column is read outin sequence to produce an overall digitized image, described in greaterdetail below.

In general, the fabrication of an STI region 32 includes etching atrench into substrate 14 and filling the trench with a dielectric toprovide a physical and electrical barrier between adjacent pixels.Refilled trench structures, for example, STI region 32, are formed byetching a trench by a dry anisotropic or other etching process and thenfilling it with a dielectric such as a chemical vapor deposited (CVD) orhigh density plasma (HDP) deposited silicon oxide or silicon dioxide(SiO₂). The filled trench is then planarized by a chemical mechanicalplanarization (CMP) or etch-back process so that the dielectric remainsonly in the trench and its top surface remains level with that of thesilicon substrate.

Forming pixel cell 10 between STI regions 32, however, creates problemsin the operation of the pixel cell 10. For example, STI sidewalls andbottom portion, herein collectively referred to as STI boundaries 32 a,have a higher silicon density than the substrate 14, creating a higherdensity of “trap sites” along the STI boundaries 32 a as compared to thesilicon/gate oxide interface of a transistor (e.g., transfer transistor16). Trap sites are areas in the silicon dioxide/silicon interface thatcan “trap” electrons or holes. Trap sites result from defects along thesilicon dioxide/silicon interface between the STI boundaries 32 a andthe silicon substrate 14. For example, dangling bonds or broken bondsalong the silicon dioxide/silicon interface can trap electrons or holes.

The trap sites are typically uncharged, but become energetic whenelectrons and holes become trapped therein. Highly energetic electronsor holes are called hot carriers. Hot carriers can get trapped in theavailable trap sites, and contribute to the fixed charge of the deviceand change the threshold voltage and other electrical characteristics ofthe device. STI boundaries 32 a may also contain a higher level ofdefect density due to different crystallographic orientation planesalong the STI boundaries 32 a. The high defect densities along withhigher trap sites lead to higher leakage levels along the STI boundaries32 a. The current generation from trap sites inside or near thephotosensor 12 contributes to dark current (i.e., electrical current inthe photosensor in the absence of light) in CMOS imagers since aconstant charge is leaking into the photosensor 12. Dark current isdetrimental to the operation and performance of a photosensor.Accordingly, it is desirable to provide an isolation technique thatprevents current generation or current leakage.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an isolation technique for preventingcurrent generation and leakage in a pixel cell of an imager device.

The above and other features and advantages are achieved in variousembodiments of the invention by providing a pixel cell having ahalogen-rich region localized between an isolation region and aphotosensor. The halogen-rich region prevents leakage from the isolationregion into the photosensor, thereby suppressing the dark current in theimager.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-described features and advantages of the invention will bemore clearly understood from the following detailed description, whichis provided with reference to the accompanying drawings in which:

FIG. 1 illustrates a partial cross-sectional representation of aconventional pixel cell;

FIG. 2 illustrates a partial cross-sectional representation of a pixelcell constructed in accordance with an exemplary embodiment of theinvention;

FIGS. 3-6 illustrate stages in fabrication of the pixel cell illustratedin FIG. 2;

FIG. 7 illustrates a partial cross-sectional representation of a pixelcell constructed in accordance with a second exemplary embodiment of theinvention;

FIG. 8 illustrates a partial cross-sectional representation of a pixelcell constructed in accordance with a third exemplary embodiment of theinvention;

FIG. 9 illustrates a partial cross-sectional representation of a pixelcell constructed in accordance with a fourth exemplary embodiment of theinvention;

FIG. 10 illustrates a partial cross-sectional representation of a pixelcell constructed in accordance with a sixth exemplary embodiment of theinvention;

FIG. 11 is a block diagram of a CMOS imager incorporating at least onepixel cell constructed in accordance with an embodiment of theinvention; and

FIG. 12 is a schematic diagram of a processor system incorporating theCMOS imager of FIG. 11 in accordance with an exemplary embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof and show by way ofillustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized, and thatstructural, logical, and electrical changes may be made withoutdeparting from the spirit and scope of the present invention. Theprogression of processing steps described is exemplary of embodiments ofthe invention; however, the sequence of steps is not limited to that setforth herein and may be changed as is known in the art, with theexception of steps necessarily occurring in a certain order.

The terms “semiconductor substrate,” “silicon substrate,” and“substrate” are to be understood to include any semiconductor-basedstructure. The semiconductor structure should be understood to includesilicon, silicon-on-insulator (SOI), silicon-on-sapphire (SOS),silicon-germanium, doped and undoped semiconductors, epitaxial layers ofsilicon supported by a base semiconductor foundation, and othersemiconductor structures. When reference is made to the substrate in thefollowing description, previous process steps may have been utilized toform regions or junctions in or over the base semiconductor orfoundation.

The term “pixel,” as used herein, refers to a photo-element unit cellcontaining a photosensor for converting photons to an electrical signal.For purposes of illustration, a single representative pixel and itsmanner of formation are illustrated in the figures and descriptionherein; however, typically fabrication of a plurality of like pixelsproceeds simultaneously. Accordingly, the following detailed descriptionis not to be taken in a limiting sense, and the scope of the presentinvention is defined only by the appended claims.

The term “halogen-rich region,” as used herein, refers to an ion-richregion in a substrate. The ions formed in the substrate may include anyof the halogen ions including, but not limited to, fluorine, chlorine,bromine, iodine, or any combination thereof.

In the following description, the invention is described in relation toa CMOS imager for convenience; however, the invention has widerapplicability to any photosensor of any imager cell. For example,although illustrated as a pinned photodiode, photosensor 12 (FIG. 2)could be a p-n junction photodiode, a Schottky photodiode, a photogate,or any other suitable photoconversion device. Additionally, althoughdescribed in relation to a CMOS imager, the invention is applicable to acharge-coupled-device (CCD).

Referring now to the figures, where like reference numbers designatelike elements, FIG. 2 illustrates an exemplary pixel cell 100constructed in accordance with an embodiment of the invention. Pixelcell 100 is similar to the FIG. 1 pixel cell with the significantimprovement of having a halogen-rich region 34 localized around the STIboundaries 32 a that contact the silicon substrate 14. As noted above,the halogen-rich region 34 could include any halogen ion, including, butnot limited to, fluorine, chlorine, bromine, iodine, or any combinationthereof.

According to the invention, the halogen-rich region 34 has aconcentration of halogen ions in a range of about 1×10¹³/cm³ to about1×10¹⁶/cm³ with a peak ion concentration in the range of about 300-800Å. In the illustrated embodiment, the halogen-rich region 34 has ahalogen ion concentration of about 1×10¹⁴/cm³ and a substantiallyhomogenous ion concentration to a depth of about 500 Å.

Although the halogen-rich region 34 may not decrease the number ofdefects found in the silicon dioxide/silicon interface between STIboundaries 32 a and the substrate 14, the halogen-rich region 34 willprevent the effects of current generation or current leakage found inconventional pixel cells (described above with respect to FIG. 1). Thehalogen-rich region 34 acts to compensate the charge associated withdangling bonds or broken bonds near the silicon dioxide/siliconinterface. By compensating for the charge associated with the danglingbonds, the resulting pixel cell 100 has decreased current generationand/or current leakage and, therefore, suppressed dark current.

FIGS. 3-6 illustrate stages of an exemplary method of forming the FIG. 2pixel cell 100 by ion implantation. FIG. 3 illustrates a substrate 14having trenches 36 formed by conventional etching methods. For example,the trenches 36 can be etched by chemical etching, anisotropic etching,reactive ion etching (RIE), or other means of creating a trench in thesubstrate 14. Masks 38 are positioned on the substrate 14 such that thetrenches 36 and desired surfaces of the substrate 14 adjacent to thetrenches 36 are exposed. The masks 38 serve to protect the surfaces ofthe substrate 14 that the masks 38 cover from halogen ion implantation,illustrated as arrows in FIG. 3. The halogen ion implantation results inlocalized halogen-rich regions 34 in the substrate 14.

According to an exemplary embodiment of the invention, the halogen-richregion 34 may be doped with an implant energy in the range of about 10keV to about 50 keV to achieve a depth from a surface of the substrate14 in the range of about 300 Å to about 800 Å. The illustrated pixelcell 100 is implanted with an implant energy of about 25 keV to achievea peak halogen ion concentration at a depth of about 500 Å. As discussedbelow with respect to Table 1, the selected ion energy results in a peakconcentration of halogen ions at a particular depth from the surface ofthe substrate 14. Table 1 illustrates the different ion energy ranges(Ion Energy) for implantation conditions using fluorine as the halogenspecies; the depth of peak concentration at a given ion energy (Range);the vertical standard deviation from the peak concentration depth(Longitudinal Straggling); and the horizontal standard deviation fromthe area of peak concentration (Lateral Straggling).

Ion Longitudinal Lateral Energy Range Straggling Straggling (keV)(Angstroms) (Angstroms) (Angstroms) 10.00 233 123 88 11.00 253 132 9512.00 274 141 102 13.00 295 150 108 14.00 315 159 114 15.00 336 168 12116.00 357 177 127 17.00 377 185 133 18.00 398 194 139 20.00 440 211 15122.00 482 227 163 24.00 524 243 175 26.00 566 259 187 28.00 608 275 19930.00 651 290 210 33.00 715 313 228 36.00 779 335 245 40.00 866 364 26845.00 974 399 296 50.00 1083 433 324 55.00 1192 466 351 60.00 1302 498378 65.00 1411 529 405 70.00 1521 559 431 80.00 1741 616 483 90.00 1961671 534 100.00 2180 723 583

FIG. 4 illustrates the formation of an STI region 32 in the substrate14. Specifically, the trenches 36 are filled with a dielectric,including, but not limited to, a chemical vapor deposited (CVD) siliconoxide or silicon dioxide or high density plasma (HDP) deposited siliconoxide or silicon dioxide. The dielectric-filled trench is thenplanarized by chemical mechanical planarization (CMP) or etch-backprocess such that the dielectric remains only in the trenches 36, andthe STI region 32 has a top surface that is level with that of thesubstrate 14.

FIG. 5 illustrates the formation of a photosensor 12 having a p-region12 a and an n-region 12 b formed in the substrate 14. It should be notedthat although the photosensor is illustrated and described withreference to a p-n-p photodiode, it is not intended to limit theinvention to such a photosensor. For example, the described photosensor12 could be an n-p-n photodiode, a photogate, or any suitablephotoconversion device capable of converting light to an electricalcharge. FIG. 5 also illustrates the formation of a more heavily dopedp-type well 20 in substrate 14. Additionally, a transfer transistor gate16 a and a reset transistor gate 22 a are formed by conventionalmethods. It should be noted that transfer transistor gate 16 a is formedonly in a pixel cell 100 having a four-transistor (4T) configuration.Transfer transistor 16 (FIG. 6) is an optional transistor in pixel cell100, and pixel cell 100 could have a three-transistor (3T)configuration, without a transfer transistor. Alternatively, pixel cell100 could have more than four or less than three transistors.

FIG. 6 illustrates the formation of gate stack sidewall insulators 16 b,22 b on the sides of the gate stacks 16 a, 22 a, together formingtransfer transistor gate 16 and reset transistor gate 22, respectively.FIG. 6 also illustrates the formation of n-type regions within p-typewell 20; specifically, floating diffusion region 18 and source/drainregion 30. Readout circuitry (shown schematically) is also formed, andincludes source follower transistor 24, row select transistor 26, andcolumn line 28.

FIGS. 3-6 illustrate only one exemplary method of forming thehalogen-rich region 34 in the substrate 14, and is not intended to belimiting. Other methods of forming the halogen-rich region 34 within thesubstrate 14 may also be employed, resulting in the pixel cell 100illustrated in FIG. 2. For example, halogen species may be incorporatedthrough a high density plasma (HDP) deposition process. Another methodof incorporating halogen species into the substrate 14 is bysolid-source diffusion.

The illustrated pixel cell 200 of FIG. 7 is a second embodiment of theinvention, in which a halogen-rich region 34 is localized between STIregion 32 and photosensor 12. Because most leakage occurs from STIsidewall 32 b, specifically that portion of the STI sidewall 32 b incontact with the p-region 12 a of the photosensor 12, the halogen-richregion 34 can be localized between the photosensor 12 and STI sidewalls32 b, and still serve to suppress dark current.

Another region contributing to leakage is the STI bottom portion 32 c,illustrated in FIG. 8. In the illustrated pixel cell 300 of FIG. 8, thehalogen-rich region 34 is localized underneath the STI region 32 inaddition to being localized between the photosensor 12 and the STIsidewall 32 b. By forming the halogen doped region 34 underneath the STIregion 32, the halogen-rich region 34 may counter any trap sites locatedin the STI bottom portion 32 c, thereby suppressing any dark current inthe pixel cell 300.

Referring to FIG. 9, a halogen-rich region 34 is formed within a topregion 40 of the substrate 14. Forming the halogen-rich region 34 withinthe entire top region 40 of the substrate 14, not only counters theeffects of defects found in the silicon dioxide/silicon interfacebetween the STI boundaries 32 a and the substrate 14, as discussed abovewith respect to FIGS. 2-8, but also counters the negative effects ofdefects found in the silicon/gate oxide interface 42 between thetransfer transistor gate 16 a (FIG. 5) and the substrate 14, or thereset transistor gate 22 a (FIG. 5) and the substrate 14. As discussedabove with respect to FIG. 1, the silicon/gate oxide interface 42contains defects that create trap sites along the gate oxide (e.g.,transfer transistor gate 16 a of FIG. 5) and the substrate 14.Halogen-rich region 34 counters the detrimental effects of the trapsites, resulting in a pixel cell 400 that prevents current generation orcurrent leakage.

FIG. 10 illustrates a pixel cell 500 in which a trench 36 (FIG. 4) islined with a halogenated low constant dielectric material 46 prior tofilling the trench 36 with a CVD or HDP deposited silicon oxide orsilicon dioxide. For example, the trench 36 may be lined withfluorinated silicon oxide (SiOF), and then filled with a CVD siliconoxide material to form an STI region 32. By lining the trench 36 with ahalogenated low constant dielectric material 46, e.g., SiOF, the halogendiffuses into the substrate 14, forming a halogen-rich region 34 betweenthe STI region 32 and the substrate 14. Although illustrated as a thinmaterial, it should be noted that the halogenated low constantdielectric material 46 could fill the entire trench, forming an STIregion formed of, for example, SiOF.

The pixel cells 100, 200, 300, 400, 500 of the invention may be combinedwith peripheral circuitry to form an imager device. For example, FIG. 11illustrates a block diagram of a CMOS imager device 908 having a pixelarray 900. Pixel array 900 comprises a plurality of pixels arranged in apredetermined number of columns and rows. The illustrated pixel array900 contains at least one pixel cell 100, 200, 300, 400, 500 constructedin accordance with any one of the exemplary embodiments of the inventionas described above with respect to FIGS. 2-10. For clarity's sake, theCMOS imager 908 of FIG. 11 is now discussed as incorporating at leastone pixel cell 100 of FIG. 6; however this is not intended to limit theCMOS imager 908 to such an embodiment.

The pixel cells 100 of each row in array 900 are all turned on at thesame time by a row select line, and the pixel cells 100 of each columnare selectively output by respective column select lines. A plurality ofrows and column lines are provided for the entire array 900. The rowlines are selectively activated in sequence by the row driver 910 inresponse to row address decoder 920 and the column select lines areselectively activated in sequence for each row activation by the columndriver 960 in response to column address decoder 970. The CMOS imager908 is operated by the control circuit 950, which controls addressdecoders 920, 970 for selecting the appropriate row and column lines forpixel readout, and row and column driver circuitry 910, 960 to applydriving voltage to the drive transistors of the selected row and columnlines.

The pixel output signals typically include a pixel reset signal V_(rst)taken from the floating diffusion node (e.g., 18 of FIG. 6) when it isreset and a pixel image signal V_(sig), which is taken from the floatingdiffusion node (e.g., 18 of FIG. 6) after charges generated by an imageare transferred to the node. The V_(rst) and V_(sig) signals are read bya sample and hold circuit 961 and are subtracted by a differentialamplifier 962, which produces a difference signal (V_(rst)−V_(sig)) foreach pixel cell 100, which represents the amount of light impinging onthe pixels. This signal difference is digitized by an analog-to-digitalconverter 975. The digitized pixel difference signals are then fed to animage processor 980 to form a digital image. In addition, as depicted inFIG. 11, the CMOS imager device 908 may be included on a singlesemiconductor chip (e.g., wafer 1100).

FIG. 12 shows system 1000, a typical processor based system modified toinclude the imager device 908 illustrated in FIG. 11. Processor basedsystems exemplify systems of digital circuits that could include animager device 908. Examples of processor based systems include, withoutlimitation, computer systems, camera systems, scanners, machine visionsystems, vehicle navigation systems, video telephones, surveillancesystems, auto focus systems, star tracker systems, motion detectionsystems, image stabilization systems, and data compression systems forhigh-definition television, any of which could utilize the invention.

System 1000 includes an imager device 908 having the overallconfiguration depicted in FIG. 11 with pixels of array 900 constructedin accordance with any of the various embodiments of the invention.System 1000 includes a processor 1002 having a central processing unit(CPU) that communicates with various devices over a bus 1004. Some ofthe devices connected to the bus 1004 provide communication into and outof the system 1000; an input/output (I/O) device 1006 and imager device908 are examples of such communication devices. Other devices connectedto the bus 1004 provide memory, illustratively including a random accessmemory (RAM) 1010, hard drive 1012, and one or more peripheral memorydevices such as a floppy disk drive 1014 and compact disk (CD) drive1016. The imager device 908 may receive control or other data from CPU1002 or other components of system 1000. The imager device 908 may, inturn, provide signals defining images to processor 1002 for imageprocessing, or other image handling operations.

It should be noted that although the invention has been described withspecific references to CMOS pixel cells having a halogen-rich region 34(FIGS. 2-10) formed between a photosensor 12 and STI region 32, theinvention has broader applicability and may be used in any imagingapparatus. For example, the present invention may be used in conjunctionwith charge-coupled-device (CCD) imagers. The above description anddrawings illustrate preferred embodiments which achieve the objects,features, and advantages of the present invention. Although certainadvantages and preferred embodiments have been described above, thoseskilled in the art will recognize that substitutions, additions,deletions, modifications and/or other changes may be made withoutdeparting from the spirit or scope of the invention. Accordingly, theinvention is not limited by the foregoing description but is onlylimited by the scope of the appended claims.

1. A method of forming a pixel cell, said method comprising the acts of:forming a photosensor having a first doped region and a second dopedregion in association with a semiconductor substrate, said photosensorcapable of generating dark current; forming a trench in saidsemiconductor substrate adjacent to said photosensor; forming ahalogen-rich region localized at least at a sidewall region of saidtrench, said halogen-rich region having a halogen concentrationsufficient to reduce said dark current; and filling said trench with adielectric material.
 2. The method of claim 1, wherein said act offorming a halogen-rich region comprises doping said substrate with anion selected from the group consisting of fluorine, chlorine, bromine,iodine, and any combination of fluorine, chlorine, bromine and iodine.3. The method of claim 2, wherein said act of doping said substrate isperformed by ion implantation.
 4. The method of claim 2, wherein saidact of doping said substrate is performed by incorporating halogenthrough a high density plasma deposition process.
 5. The method of claim2, wherein said act of doping said substrate is performed by solidsource diffusion of halogen ions.
 6. The method of claim 1, wherein saidhalogen-rich region is formed at a depth of about 300 Å to about 800 Åfrom a surface of said semiconductor substrate.
 7. The method of claim1, wherein said halogen-rich region is formed having a concentration ofhalogen from about 5×10¹³/cm³ to about 5×10¹⁵/cm³.
 8. The method ofclaim 1, wherein said first doped region of said photosensor is formedto overlap with said halogen-rich region.
 9. The method of claim 1,further comprising the act of providing a mask over said semiconductorsubstrate before said step of forming said halogen-rich region.
 10. Themethod of claim 1, further comprising the act of forming a chargecollection region in said semiconductor substrate.
 11. The method ofclaim 10, further comprising the act of forming a transfer transistorbetween said photosensor and said charge collection region.
 12. Themethod of claim 1, wherein said act of forming a halogen-rich regioncomprises doping said substrate with an ion selected from the groupconsisting of chlorine, bromine, iodine, and any combination ofchlorine, bromine, and iodine.